Check to see if you are getting spark and fuel to the engine. One will be missing. At that point, you need to focus on that system.
If the starter isn't working, make sure the battery is fully charged, the terminals are clean and tight, and have a helper turn the key to the start position while you check for 12V at the smaller wire on the starter. If there is 12v, replace the starter. If there isn't, check the starter relay and work your way back to the ignition switch.